1. Ontology is concerned with the nature of being or existence.
The late W.V. Quine began his famous essay "On What There Is" with these words:
A curious thing about the ontological problem is its simplicity. It can be put in three Anglo-Saxon monosyllables: 'What is there?' It can be answered, moreover, in a word - 'Everything' - and everyone will accept this answer as true. However, this is merely to say that there is what there is. There remains room for disagreement over cases; and so the issue has stayed alive down the centuries. (Quine  1980, 1)
2. Michael Horton believes that "epistemology follows ontology. In other words, our theory of how we know anything depends on what we think there is to be known." (2011, 47)
So Horton began "Chapter One - Dissonant Dramas: Paradigms for Knowing God and the Word" of his The Christian Faith: A Systematic Theology for Pilgrims On the Way with two sections on ontology.
In "section I - Dissonant Dramas: The Nature of Reality", Horton lays out the ontology of four anti-Biblical worldview paradigms: pantheism, panentheism, deism and atheism.
Using a metaphor of Paul Tillich’s, Horton group pantheism and panentheism under "overcoming estrangement".
Using another metaphor, Horton group deism and atheism under "the stranger we never meet".
3. In "section II - A Covenantal Account of 'Meeting a Stranger'", Horton lays out a Biblical ontology.
The Biblical ontology is based on the doctrines of God and creation: God created the world.
So being or existence is of one of two types: Uncreated and created.
"This model assumes that God and the world are distinct - Creator and creation. The world is dependent on God, but God is independent of the world." (2011, 41)
Hence, the Creator-creation Distinction.
So far so good.
Then Horton tries to introduce the idea of "covenant" into ontology and his narrative goes off in a tangent.
4. As a Reformed believer, I like covenant.
Covenant is one of the organizing principles of the Bible and is very important.
But covenant has no obvious bearing on a theory of ontology.
What there is depends on the nature of God and his acts of creation.
Covenant between God and man comes after creation.
Horton himself even tells us that "the history of the covenantal relationship of God and humanity rather than the metaphysics of being and becoming is the interest of this model." (2011, 42)
I do not know in what sense Horton's "covenantal ontology" (2011, 44) answers the question 'What is there?'
What Horton has written in section II is true and edifying, but mostly irrelevant to the topic at hand.
5. I suspect what happens is that Horton is forced off-course by his narrative approach to systematic theology.
Systematic theology is a synthesis and presentation of the Bible in a topical and logical arrangement.
Horton chosen literary form (narrative) is a mismatch for the content of his systematic theology (topical and logical presentation of the Bible).
Narrative, being an account of events, is ideal for presenting history but not systematic theology.
Is that why Horton writes that "the history of the covenantal relationship of God and humanity rather than the metaphysics of being and becoming is the interest of this model." (2011, 42)
If so, then Horton's systematic theology is straightjacketed by his narrative approach.
Like systematic theology, ontology is susceptible to a topical and logical presentation but not a narrative one.
Horton, Michael. 2011. The Christian Faith: A Systematic Theology for Pilgrims On the Way. Grand Rapids, Michigan: Zondervan.
Quine, Willard Van Orman.  1980. On What There Is. Reprinted in From a Logical Point of View: Nine Logico-Philosophical Essays, Second Edition, Revised, 1-19. Cambridge, Massachusetts: Harvard University Press.